Dns

DNS — Domain Name System > Exam relevance: DNS appears in the IP Services domain (10% of the exam) and is tested almost exclusively through multiple-choice questions. Expect 1–3 questions focused on how DNS resolves names, key record types, and how Cisco devices are configured to use DNS. --- ## What is DNS and Why Does It Exist? Every device on an IP network communicates using IP addresses, but humans remember names — not numbers. DNS (Domain Name System) is the distributed hierarchical system that translates human-readable hostnames (like www.cisco.com) into IP addresses that routers and hosts can actually use. Without DNS, every user would need to manually type an IP address to reach any resource. DNS automates that lookup transparently in the background. --- ## How DNS Resolution Works When a host needs to resolve a name to an IP address, the following process occurs: 1. The host checks its local DNS cache first. 2. If not cached, the host sends a DNS query (UDP port 53) to its configured DNS server. 3. The DNS server either answers directly (from its own cache or zone data) or performs recursive resolution by querying other DNS servers on behalf of the client. 4. The resolved IP address is returned to the host and cached for future use. > Key detail: DNS uses UDP port 53 for standard queries. TCP port 53 is used for large responses (zone transfers). For the CCNA exam, remember UDP/53. --- ## DNS Record Types The CCNA exam expects you to recognize the most common DNS record types: | Record Type | Purpose | Example | |-------------|---------|---------| | A | Maps a hostname to an IPv4…